The present invention relates generally to devices or rigs for driving piles and the like into the ground and for extracting piles that have been driven into the ground, and more particularly to rigs of this type that are provided with a vibration/tension load suppressor for absorbing vibration and tension loads generated by the driving and extracting operations.
Vibratory pile driver/extractors which are used for driving and extracting piles and the like generally include a vibratory unit that is connected to a housing which in turn is connected to a cable or other support that lowers and lifts the housing and vibratory unit during the driving and extracting operations, respectively. The vibratory unit is connected to the pile and usually includes rotating eccentrics that impart vibrations to the pile to assist in driving and extracting the pile.
During driving operations, the housing and vibratory unit are positioned on top of the pile itself and to a large extent are supported thereby. Therefore, there is little or no tension load on the cable as it lowers the housing and vibratory unit while the pile is driven into the ground. However, during pile extracting operations, the cable must not only bear the dead weight of the housing and vibratory unit, it is also subjected to the high tension loads required to extract the pile from the ground. Vibration and tension loads generated during the pile extraction operation can damage the supporting cable and hoisting machinery if the vibration and tension loads are not properly absorbed.
In early designs, springs were provided as vibration/tension load absorbers, but because the tension loads varied so much, depending on whether the rig was driving or extracting piles, it was necessary to manually adjust the stiffness of the springs each time the rig changed from pile driving and pile extracting operations, and vice versa. This arrangement had the major disadvantage of being both time consuming and labor intensive.
Another vibration/tension load absorbing arrangement is disclosed in Herz U.S. Pat. No. 3,502,160. To avoid the problem of having to manually adjust the vibration/tension load absorbing springs, this patent utilizes two different sets of compression springs acting between a yoke and the vibratory unit or body, with one set having a greater stiffness than the other. The first set of weaker springs is connected between the yoke and the vibratory unit and is designed to maintain the yoke and the vibratory unit in fixed relation to one another during pile driving operations when there is little or no tension load on the cable. The second set of springs is stiffer than the first set, whereby during pile extracting operations the higher tension load on the cable is taken over by the set of stiffer springs when the tension load reaches a predetermined level. One disadvantage of this arrangement is that during pile driving operations, when there is generally some tension maintained in the cable, the vibration imparted to the cable is not absorbed since the first set of weaker springs maintains the yoke and the vibratory unit in fixed, rigid relation to one another. Additionally, during pile extracting operations, when the tension load exceeds the level at which the set of stiffer springs come into play, the first set of weaker springs become essentially non-functional and do not supplement the tension absorbing action of the set of stiffer springs.
Kniep U.S. Pat. No. 3,865,501 discloses a soil compactor that is operated by a vibratory unit carried on a housing by multiple compression springs that appears to be generally similar to the Herz arrangement, but since the Kniep patent does not include any detailed description of the compressions springs and their operation, it is not clear exactly how the multiple springs operate.
In White U.S. Pat. Nos. 5,117,925 and 5,263,544, pile driving and extracting rigs are disclosed which include multi-stage vibration/tension load absorbing elements for absorbing the vibration generated by a vibratory unit and tension loads. In these rigs, some of the vibration/tension load absorbing elements are connected between a base section and an intermediate section, and others of the vibration/tension load absorbing elements are connected between the intermediate section and a connection section. All of the vibration/tension load absorbing elements are in the form of horizontally extending elastomeric members arranged to absorb the loads by shear strain. The elastomeric members have varying thicknesses, and the vibration loads and tension loads are absorbed in stages. While these shear members are capable of absorbing loads encountered in conventional pile driving and pile extracting rigs, each member has a somewhat limited vibration and tension load absorbing capacity, and therefore in instances where the tension loads are very high, the number of vibration/tension load absorbing elements must be increased, all of which can increase the expense of the rig and the size of rig that must accommodate the increased number of shear members.
In accordance with the present invention, an apparatus for driving and extracting piles is provided which overcomes drawbacks of known devices of this type, and which provides an improved arrangement for absorbing vibration and tension loads associated with devices of this type.